This is a randomized, double-blind, placebo-controlled, multicenter, parallel group comparison study designed to evaluate the safety and efficacy of chronic, passive antibody immunotherapy employing one of two doses of a mucoid exopolysaccharide Pseudomonas aeruginosa intravenous immune globulin (MEP IGIV) compared to placebo (human serum albumin) in cystic fibrosis (CF) patients with endobronchial colonization by mucoid Pseudomonas aeruginosa. The study will evaluate the frequency of acute pulmonary exacerbations over a one-year period.